The present invention relates to a method for manufacturing a solid-state imaging device.
In recent years, it is common to provide solid-state imaging devices with chargecoupled devices (hereinafter xe2x80x9cCCDxe2x80x9d will be referred to) used for transferring charge. This solid-state imaging device has a configuration in which a plurality of light-receiving portions are arranged in a matrix, and a charge transfer portion is formed corresponding to each line of the matrix. The charge transfer portion is a CCD in which a transfer channel portion is formed in a silicon substrate and a transfer electrode is formed above the transfer channel portion via a gate insulating film. In such a solid-state imaging device, in order to improve the sensitivity by suppressing the reflection on the surface of the light-receiving portion, it has been proposed to form an anti-reflection film above the light-receiving portion.
FIGS. 5A to 5C are cross-sectional views to illustrate steps of a method for manufacturing a solid-state imaging device provided with an anti-reflection film. First, on the silicon substrate 50 provided with a light-receiving portion 52 and a transfer channel portion 51, a silicon oxide film 53, a silicon nitride film 54 and a silicon oxide film 55 are formed in this order, thereby forming a three-layered gate insulating film (see FIG. 5A). Then, a polysilicon film is formed, followed by patterning thereof by photolithography and etching, thereby forming a transfer electrode 56 above the transfer channel portion 51 (FIG. 5B). Next, the surface of the transfer electrode 56 is covered with a silicon oxide film 57 by thermal oxidization, followed by patterning of the silicon nitride film 54, thereby forming an anti-reflecting film 54a above the light-receiving portion 52 (FIG. 5C).
FIGS. 6A to 6E are cross-sectional views to illustrate steps of another method for manufacturing a conventional solid-state imaging device. Similar to FIGS. 5A to 5B, on a silicon substrate 60 provided with a light-receiving portion 62 and a transfer channel portion 61, a three-layered gate insulating film including a silicon oxide film 63, a silicon nitride film 64 and a silicon oxide film 65, and a transfer electrode 66 are formed (see FIG. 6A and FIG. 6B). Then, after a silicon oxide film 67 is formed on the surface of the transfer electrode 66, the silicon nitride film 64 above the light-receiving portion 62 is removed (see FIG. 6C). Thereafter, a new silicon nitride film 68 is formed (see FIG. 6D), followed by patterning thereof so as to form an anti-reflecting film 68a above the light-receiving portion 62 (see FIG. 6E).
FIGS. 6A to 6F are cross-sectional views to illustrate steps of another method for manufacturing a conventional solid-state imaging device. Similar to FIGS. 5A to 5B, on a silicon substrate 60 provided with a light-receiving portion 62 and a transfer channel portion 61, a three-layered gate insulating film including a silicon oxide film 63, a silicon nitride film 64 and a silicon oxide film 65, and a transfer electrode 66 are formed (see FIG. 6A and FIG. 6B). Then, after a silicon oxide film 67 is formed on the surface of the transfer electrode 66, the silicon nitride film 64 above the light-receiving portion 62 is removed (see FIG. 6C). Thereafter, a new silicon nitride film 68 is formed (see FIG. 6D), followed by patterning thereof so as to form an anti-reflecting film 68a above the light-receiving portion 62 (see FIG. 6E).
In general, as the etching for forming the transfer electrode, dry etching is carried out. However, in the manufacturing method shown in FIGS. 5A to 5C, when the dry etching is carried out, not only the polysilicon film but also the silicon oxide film 55 and the silicon nitride film 54 above the light-receiving portion 52 are etched (see FIG. 5B). As a result, the film thickness of the silicon nitride film, that is, the anti-reflection film 54a above the light-receiving portion 52 is reduced. Since the anti-reflecting effect is determined by the refractive index and film thickness of the anti-reflection film 54a, if the film thickness of the anti-reflection film 54a is reduced due to the dry etching, the anti-reflecting effect may be deteriorated.
On the other hand, in the manufacturing method shown in FIGS. 6A to 6E, after dry etching for forming the transfer electrode, the silicon nitride film 64 above the light-receiving portion 62 is removed and then the new silicon nitride film is formed as an anti-reflection film (see FIGS. 6C to 6E). Therefore, it is possible to avoid the reduction of the film thickness of the anti-reflection film and to achieve a sufficient anti-reflecting effect. However, since a step of removing the silicon nitride film 64 and a step of forming the new silicon nitride film 68 are required, the number of steps is increased, and the manufacturing efficiency is reduced.
With the foregoing in mind, it is an object of the present invention to provide a method capable of efficiently manufacturing a solid-state imaging device provided with an anti-reflection film and capable of suppressing the film thickness of the anti-reflection film from being reduced due to the etching.
In order to achieve the above-mentioned objects, a method for manufacturing a solid-state imaging device includes: forming a transfer channel portion and a light-receiving portion in a silicon substrate; forming a silicon oxide film on the silicon substrate; forming a silicon nitride film on the silicon oxide film, the silicon nitride film acting as a gate insulating film together with the silicon oxide film above the transfer channel portion and acting as an anti-reflection film above the light-receiving portion; forming a protection film on the silicon nitride film; forming a polysilicon film above the silicon nitride film via the protection film at least above the light-receiving portion; and etching the polysilicon film so as to form a transfer electrode above the transfer channel portion; wherein the etching of the polysilicon film is carried out so that the polysilicon film is removed above the light-receiving portion while the protection portion remains above the light-receiving portion.
According to such a manufacturing method, since the silicon nitride film constituting the gate insulating film and the silicon nitride film constituting the anti-reflecting film are formed in the same step, a solid-state imaging device provided with an anti-reflection film can be manufactured efficiently. Furthermore, in the etching for forming a transfer electrode, above the light-receiving portion, since the protection film is present on the silicon nitride film, it is possible to suppress the reduction of the film thickness of the silicon nitride film (i.e., anti-reflection film). Therefore, it is possible to form the anti-reflection film having a film thickness serving the anti-reflecting purpose, and to manufacture a solid-state imaging device that is excellent in sensitivity.
In the above-mentioned manufacturing method, it is preferable that the film thickness of the protection film is in the range from 5 nm to 100 nm at least above the light-receiving portion. It is preferable because it is possible to sufficiently suppress the reduction of the film thickness of the anti-reflection film due to the etching for forming the transfer electrode.
Furthermore, in the above-mentioned manufacturing method, it is preferable that the film thickness of the silicon nitride film is in the range from 5 nm to 100 nm at least above the light-receiving portion. It is preferable because a further excellent anti-reflecting effect can be achieved.
Furthermore, in the above-mentioned manufacturing method, it is preferable that the protection film is thinned or removed at least above the transfer channel portion before the polysilicon film is formed. It is preferable because a large transfer capacity can be secured in the charge transfer portion formed of the transfer channel portion and the transfer electrode.
With such a preferable embodiment, it is preferable that the protection film is thinned or removed above the transfer channel portion, and at least above a part between the transfer channel portion and the light-receiving portion. By thinning or removing the protection film in a region between the transfer channel portion and the light-receiving portion, the voltage for reading out the charge from the light-receiving portion to the transfer channel portion can be lowered.
Furthermore, with such a preferable embodiment, it is preferable that the film thickness of the thinned part of the protection film is 1 nm to 50 nm.
Furthermore, with such a preferable embodiment, it is preferable that a new insulating film is formed after the protection film is removed at least above the transfer channel portion before the polysilicon film is formed. In this case, it is preferable that the film thickness of the new insulating film is 1 nm to 50 nm.
In the above-mentioned manufacturing method, it is preferable that the film thickness of the silicon oxide film is allowed to be different between a part above the transfer channel portion and a part above the light-receiving portion before the silicon nitride film is formed. In this case, it is preferable that the film thickness of the silicon oxide film is 1 nm to 80 nm above the transfer channel portion and 1 nm to 100 nm above the light-receiving portion.
The film thickness of the silicon oxide film suitable for the gate insulating film is determined from the viewpoint of securing a sufficient withstand voltage and transfer capacity. On the other hand, from the viewpoint of anti-reflection, the suitable film thickness of the silicon oxide is determined by the relationship between the refractive index of the silicon substrate and the silicon oxide film and light entering the light-receiving portion. Therefore, both of the above-determined film thicknesses are not necessarily the same. However, with such a preferable embodiment, by adjusting the silicon oxide film so as to have the film thickness suitable for the gate insulating film above the transfer channel portion and to have the film thickness serving the anti-reflecting purpose above the light-receiving portion, it is possible to realize the improvement of sensitivity by the anti-reflection more sufficiently while securing a sufficient withstand voltage and transfer capacity in the charge transfer portion.
Furthermore, in the above-mentioned manufacturing method, it is preferable that the film thickness of the silicon nitride film is allowed to be different between a part above the transfer channel portion and a part above the light-receiving portion before the polysilicon film is formed. In this case, it is preferable that the film thickness of the silicon nitride film is 1 nm to 80 nm above the transfer channel portion and 5 nm to 100 nm above the light-receiving portion.
Similar to the above-mentioned silicon oxide film, in the silicon nitride film, the film thickness suitable for the gate insulating film and the film thickness suitable for the anti-reflection film are not necessarily the same. However, with such a preferable embodiment, the silicon nitride film can be adjusted so as to have the film thickness suitable for the gate insulating film above the transfer channel portion and to have the film thickness suitable for the anti-reflection film above the light receiving portion. Thus, it is possible to realize the improvement of the sensitivity by the anti-reflection more sufficiently while securing a sufficient withstand voltage and transfer capacity in the charge transfer portion.
Furthermore, in the above-mentioned manufacturing method, an additional insulating film is formed on the silicon nitride film at least above the light-receiving portion after the silicon nitride film is formed and before the protection film is formed. This additional insulating film works as an anti-reflection film together with the silicon nitride film. For example, a silicon nitride film can be used. Furthermore, the additional insulating film may be a multilayer film.
With such a preferable embodiment, it is easy to obtain the anti-reflection film having a film thickness serving the anti-reflecting purpose. Also, it is possible to realize the improvement of the sensitivity by the anti-reflection effect sufficiently.
In this case, it is preferable that the film thickness of the additional insulating film is allowed to be different between a part above the transfer channel portion and a part above the light-receiving portion before the polysilicon film is formed. At this time, it is preferable that the film thickness of the additional insulating film is 1 nm to 50 nm above the transfer channel portion and 1 nm to 100 nm above the light-receiving portion.
The total film thickness of the silicon nitride film and the additional insulating film formed on the silicon nitride film can be adjusted to the film thickness suitable for the gate insulating film above the transfer channel portion and to the film thickness suitable for the anti-reflection film. Thus, it is possible to realize the improvement of the sensitivity by the anti-reflection more sufficiently while securing a sufficient withstand voltage and transfer capacity in the charge transfer portion.